Attenuated derivatives of Salmonella enterica are attractive vehicles for the delivery of heterologous antigens, such as tumor antigens or tumor stroma antigens, to the mammalian immune system. S. enterica strains can potentially be delivered via mucosal routes of immunization, i.e. orally or nasally, which offers advantages of simplicity and safety compared to parenteral administration. Furthermore, Salmonella strains elicit strong humoral and cellular immune responses at the level of both systemic and mucosal compartments. Batch preparation costs are relatively low and formulations of live bacterial vaccines are highly stable. Attenuation can be accomplished by deletion of various genes, including virulence, regulatory and metabolic genes.
Several Salmonella typhimurium strains attenuated by aro mutations have been shown to be safe and effective delivery vehicles for heterologous antigens in animal models.
Approaches of delivering DNA constructs encoding antigens, in particular VEGF receptor proteins, via live attenuated Salmonella typhimurium strains into mouse target cells are described in WO 03/073995. Niethammer et al. (Nature Medicine 2002, 8(12), 1369) describes an attenuated S. typhimurium aroA strain SL7207 harboring an expression vector encoding the murine vascular endothelial growth factor receptor 2 (VEGFR-2 or FLK-1) and its use as cancer vaccine.
There is however only one attenuated Salmonella enterica serovar strain, namely Salmonella enterica serovar typhi Ty21a (short: S. typhi Ty21a), which has been accepted for use in humans.
This well-tolerated, live oral vaccine against typhoid fever was derived by chemical mutagenesis of the wild-type virulent bacterial isolate S. typhi Ty2 and harbors a loss-of-function mutation in the galE gene, as well as other less defined mutations. It has been licensed as typhoid vaccine in many countries after it was shown to be efficacious and safe in field trials.
Angiogenesis contributes to solid tumor growth and metastasis. Compounds like bevacizumab and others, for example small molecules such as sunitinib and axitinib that specifically target the tumor neovasculature have shown efficacy in a range of tumor indications (Powles et al., Br J Cancer 2011, 104(5):741-5); Rini et al., Lancet 2011, 378:1931-1939).
Tumor neovasculature is lined with endothelial cells that overexpress vascular endothelial growth factor receptor (VEGFR) 2 and are readily accessible via the blood stream. The genetic stability of these cells and their ability to support hundreds of tumor cells per endothelial cell make them a prime target for anti-cancer therapy, be it via antibodies, tyrosine kinase inhibitors, or vaccines (Augustin, Trends Pharmacol Sci 1998, 19:216-222). Recently, T-cell based immunotherapy has gained some clinical success in prostate cancer and validated the potential of anti-cancer vaccination which was often demonstrated pre-clinically (Sharma et al., Nat Rev Cancer 2011, 11:805-812). Activating the immune system against cancer cells faces multiple challenges. For example, cancerous lesions are often polyclonal and cancer cells have the propensity to mutate. Antigen specific therapy often only results in a selection of non-antigen bearing cells. Further hurdles include tumor encapsulation and loss or down-regulation of MHC molecules. Vaccination approaches that target the tumor neovasculature should in theory overcome those hurdles.